For Democrats and Republicans alike, the U.S. Census data unveiled Tuesday had an unmistakable and familiar political message: Go West.

All five of the country’s fastest-growing states — Arizona, Idaho, Nevada, Texas and Utah — are located in the Sun Belt and Rocky Mountain regions, according to the once-in-a-decade population count.

Of the 12 House seats that will shift in next year’s redistricting process, 11 will be taken from the Northeast and Midwest. Eight of the newly created districts will end up west of there.

And all of that means the battle for political power in the 21st century will most likely be fought and won in the American West.

At least in the immediate term, Republicans are expected to have the advantage. In 2008, the Republican presidential ticket won three-quarters of the states gaining seats next year, and because of changes in population, Sen. John McCain would have won six more electoral votes from the post-census map.

In many places, Republican gains on the state legislative level will put the GOP strongly in control of redrawing the congressional map.

“In terms of the impact of the region, the West and the South: More seats equals more votes. And since most of these are going to be Republican seats, that translates into more difficulty for Democrats recapturing the House,” said University of Texas professor Bruce Buchanan.

“You’ve got a double whammy here,” he said: “Good news for Republicans, bad news for Democrats, and it multiplies doubly for Republicans.”

Over the long run, the picture is less clear. In some Western states, population growth has come as a result of Hispanic immigration and the expansion of suburbs — trends that already have made states like Nevada and Colorado more hospitable to Democrats.

“Beyond the obvious shift from region to region is the shift within regions. From what I’ve seen or heard, the growth of the suburbs pretty much continues,” said Lawrence Levy, executive dean of Hofstra’s National Center for Suburban Studies.

“Within the suburbs, you will find that new immigrants and minorities will become a bigger proportion of the population, further scrambling the politics as these folks are likely to vote Democratic,” he continued. “That should keep a lot of Sun Belt metro states in play — Arizona and Nevada especially.”

Even as the region west of the Mississippi River creeps toward battlefield status, it is also home to a pair of mega-states with deep partisan leanings: Republican Texas and Democratic California.

These two states, which have already produced five of the past eight presidential election winners, are well on their way to becoming the twin pillars of their respective parties, surpassing traditional Midwestern and East Coast political bastions.

No state expanded its electoral power more in the past decade than Texas, which gained four House seats and bulked up its national muscle to 38 electoral votes — second only to California as a presidential prize.

Republicans have captured all of the statewide offices in Texas for four consecutive election cycles and this month crossed the 100-seat supermajority threshold in the 150-member state House of Representatives.

California, which routinely racked up massive, 20-plus percent population gains for most of the 20th century, did not gain House seats this time around. Still, with 53 members of Congress and 55 electoral votes, it remains the country’s unrivaled electoral colossus.

The 2010 elections also confirmed that California is a deeply — and maybe unassailably — Democratic state. Democrats won every statewide office and gained ground in the state Legislature even amid a national conservative wave.

California has not voted for a Republican presidential candidate since George H. W. Bush in 1988, while Texas has not backed a Democrat for president since Jimmy Carter in 1976.

With the decline of historical population centers in Pennsylvania, Ohio, Michigan and elsewhere, the relative influence of these partisan megastates will only increase.

Census Director Robert Groves pointed out in a Tuesday briefing that the migration of political influence has been long in coming, noting: “The trend is a growth in seats for Western and Southern states and a tendency to lose seats from the Midwest and Northeastern states.”

That trend will be felt immediately in the 2012 presidential election and perhaps nowhere more acutely than in Ohio.

After spending the past decade alongside Florida as one of two states viewed as essential to any presidential candidate, Ohio will lose two House seats and see its electoral vote total drop from 20 to 18.

That will make faster-growing Florida an obviously superior political prize: The Sunshine State is gaining two seats next year and will have as much presidential clout as New York, with 29 electoral votes.

The other losers in the reapportionment process are largely unsurprising: New York and Pennsylvania have shed House seats in every reapportionment since World War II. Illinois, Iowa and Massachusetts have either lost seats or stayed stable in every census during that period.

The other states losing representation — Ohio, New Jersey, Michigan, Missouri and Louisiana — have all lost at least one seat since 1980.

If the outcome of the census is not exactly a surprise, it still represents the crossing of a historic threshold for Western states.

“This is now the first time in history that the number of seats held by the Midwest and Northeast is less than 40 percent of the country,” noted University of Minnesota researcher Eric Ostermeier, who authors the Humphrey Institute’s Smart Politics blog.

The implications for Congress, Ostermeier said, “all depend on how these districts are carved. The mere fact that [Republican-leaning states] are gaining seats does not automatically mean an extra seat for the GOP.”

In addition to Florida, the other non-Western states gaining representation are South Carolina and Georgia, each of which will gain one new House district.

The Census Bureau also announced Tuesday that as of April 2010, the U.S. had a population of 308.7 million.

CORRECTION: A previous version of this story misstated the last year California voted Republican in a presidential race.